The present invention relates to communication systems and is directed more particularly to communication systems in which messages are transmitted from a remote station to a central station over a power distribution line through which the central station supplies operating power to the remote station.
In operating a process having a plurality of processing stations that are located at differing distances from a central control facility, it is important that the central station have current information as to the operational status of each remote station. In a chemical processing plant, for example, it is often of vital interest for a central control facility to have current information as to the temperature, pH or pressure in each of a number of remotely located chemical processing tanks. In such systems one widely used method of communicating the needed information involves the transmission of data over one or more conductor pairs that are connected between each remote station and the central station to serve as data transmission paths. While such conductor pairs work adequately from an information transmission standpoint, their cost can be extremely high, particularly where the remote stations may be hundreds or even thousands of feet from the central station.
In those communication systems in which there already exist conductor pairs that connect the central station to the remote stations, e.g. power distribution lines through which the central station supplies a-c operating power to the remote stations, there have been developed communication systems which transmit the desired information by modulating the amplitude, frequency or phase of carrier signals that are introduced into the power distribution line. In a typical communication system of this type the desired operating power is transmitted by a-c voltages and currents having a low frequency, such as 60 Hz, while the desired information is transmitted by a-c voltages and currents having very much greater frequencies, such as ten kilohertz. While such communication systems operate satisfactorily, the costs thereof can be extremely high. One reason is that in such systems there is required, in addition to the circuitry that produces and encodes the information to be transmitted, at least one carrier frequency oscillator, a modulating circuit, and a demodulating circuit. In addition, if multiplexing techniques are used to provide a plurality of data channels, there must be provided additional oscillators, modulators and demodulators as well as a number of high-pass, low-pass, and/or band pass filters for channel separation. Where the number of remote stations and the number of channels are relatively large, the cost of such communication systems exceeds even that of providing a separate pair of conductors for each desired data channel.